1. Field of the Invention
The present invention relates to a rotating speed adjustment circuit and related control system for a heat dissipation fan, and more particularly, to a rotating speed adjustment circuit and related control system capable of avoiding vibration and noise caused by current ripples on the fan coil, and preventing inaccuracy of passive devices from affecting the rotating speed.
2. Description of the Prior Art
In modern information society, a computer system has become a necessary tool in daily life. For any kind of computer system, an operating clock of a CPU is becoming higher and higher, causing heat generated more and more. Therefore, heat dissipation has become more important. In the prior art, a fan is a main way for heat dissipation. For saving energy and reducing noise, a lot of methods are developed to control a rotating speed of a CPU fan, one of which in particular is simultaneously utilizing two sets of signal sources to control the rotating speed.
Please refer to FIG. 1, which is a schematic diagram of a fan control system 10 utilizing two sets of signal sources in the prior art. The fan control system 10 is utilized for controlling a rotating speed of a fan 108, and comprises a temperature-based rotating speed control circuit 100, a pulse width modulation (PWM) rotating speed control circuit 102, an AND gate 104 and a logic circuit 106. The temperature-based rotating speed control circuit 100 is utilized for sensing a temperature, e.g. the temperature of the air dissipated by the fan 108, to generate a temperature control signal PWMTH. The PWM rotating speed control circuit 102 is utilized for receiving a system control signal VCTR outputted by the outer control circuit, e.g. the CPU, to generate a temperature control signal PWMEXT. The AND gate 104 is utilized for performing an AND operation on the temperature control signal PWMTH and the temperature control signal PWMEXT. The logic circuit 106 can output a temperature control signal PWMOUT to perform pulse width modulation on the fan 108, so as to control the rotating speed of the fan 108 by two signal sources.
Please refer to FIG. 2A, FIG. 2B and FIG. 2C. FIG. 2A is a schematic diagram of the temperature-based rotating speed control circuit 100 shown in FIG. 1. FIG. 2B is a schematic diagram of related waveforms of the temperature-based rotating speed control circuit 100. FIG. 2C is a schematic diagram of the rotating speed corresponding to the temperature-based rotating speed control circuit 100. The temperature-based rotating speed control circuit 100 comprises a thermistor RTH, resistors R1, R2, R3, a comparator 200 and an oscillator 202. The thermistor RTH and the resistor R1 are coupled in a sequence between a reference voltage VREF and a ground, to generate a division voltage VTH. Since resistance of the thermistor RTH is negatively proportional to the temperature, the voltage VTH is lower when the temperature is higher. Besides, the resistors R2, R3 are utilized for dividing voltage to generate a voltage VMIN, which is utilized for configuring a lowest rotating speed SP_min1 to avoid the fan 108 stopped due to too low temperature. The oscillator 202 is utilized for generating an oscillating signal VOS. The comparator 200 is utilized for comparing the voltages VTH, VMIN and the oscillating signal VOS, so as to output the temperature control signal PWMTH to the AND gate 104.
As shown in FIG. 2B and FIG. 2C, when the temperature sensed by the thermistor RTH is higher than a threshold temperature T_th, that is, before time T1, the voltage VTH will be lower than the voltage VMIN, causing the duty cycle of the temperature control signal PWMTH increased, to increase the rotating speed of the fan 108, and dissipate more heat. On the contrary, when the temperature sensed by the thermistor RTH is lower than the threshold temperature T_th, that is, after time T1, the voltage VTH will be higher than the voltage VMIN, causing the duty cycle of the temperature control signal PWMTH to stay in the minimum duty cycle, so as to control the rotating speed of the fan 108 to be in the lowest rotating speed SP_min1, and avoid the fan 108 stopped due to too low temperature.
On the other hand, please refer to FIG. 3A and FIG. 3B. FIG. 3A is a schematic diagram of the PWM rotating speed control circuit 102 shown in FIG. 1. FIG. 3B is a schematic diagram of the rotating speed corresponding to the PWM rotating speed control circuit 102. The PWM rotating speed control circuit 102 comprises a transistor N1, resistors R4, R5, a comparator 300, an oscillator 302. The transistor N1 is a bipolar junction transistor, and can generate a system control signal B_VCTR opposite to the system control signal VCTR. The system control signal B_VCTR is compared with the oscillating signal VOS generated by the oscillator 302, so as to derive a temperature control signal PWMEXT. The resistors R4, R5 are utilized for adjusting the high potential voltage of the signal B_VCTR, so as to determine the lowest rotating speed SP_min2. The rotating speed is thereof shown in FIG. 3B.
Therefore, via the temperature-based rotating speed control circuit 100 and the PWM rotating speed control circuit 102, the fan control system 10 can control the rotating speed of the fan 108 by simultaneously utilizing two signal sources. However, the above-mentioned method has the following drawbacks.
First, the duty cycle of the temperature control signal PWMOUT derived by performing AND operation on the temperature control signal PWMTH and PWMEXT will not be a stable value. Instead, as shown in FIG. 4A, it will generate current ripples on the fan coil, causing problems of vibration and noise.
Second, in the temperature-based rotating speed control circuit 100, since variation of the resistance of the thermistor RTH related to the temperature is insufficient, the highest and lowest voltage ranges of the oscillator 202 are needed to be shrunk to compromise the characteristics of the thermistor RTH. However, if the highest and lowest voltage ranges are too small, the rotating speed will be affected dramatically by inaccuracy of the passive devices in the circuit.
Third, a specification of the CPU fan requires that the CPU fan must be maintain a fixed rotating speed when the duty cycle of the system control signal VCTR is less than 20%, to ensure the CPU fan to generate a basic amount of wind and reach the purposes of saving energy. However, the fan control system 10 in the prior art can not meet the specification, because the lowest rotating speed thereof changes with working temperature as shown in FIG. 4B.
Thus, for the heat dissipation fan controlled both by temperature-based and PWM signals, the industries are devoted to research a fan control system which can overcome the drawbacks mentioned above.